An antireflection film is used in various image displays, such as liquid crystal displays (LCDs), plasma display panels (PDPs), electroluminescence displays (ELDs), and cathode ray tube displays (CRTs). The antireflection film is used in an eyeglass and camera.
Multilayer antireflection films having a stack of thin transparent metal oxide layers have been commonly employed. Use of a plurality of transparent layers is for preventing reflection of light in as broad a wavelength range as possible in the visible region. Thin transparent metal oxide films are formed by chemical vapor deposition (CVD) or physical vapor deposition (PVD), particularly vacuum deposition or sputtering that are categorized as PVD techniques. While thin transparent metal oxide films possess excellent optical properties as an antireflection coat, vacuum deposition and sputtering used for forming these films achieve low productivity and are unfit for large-volume production.
Methods of forming an antireflection film by wet coating with inorganic particles have been proposed as a substitute for vapor deposition techniques. For example, JP-B-60-59250 discloses an antireflection layer containing a particulate inorganic substance and having microvoids, which is obtained by coating. The antireflection layer having microvoids is obtained by applying a coating composition and treating the coating layer with activated gas.
JP-A-59-50401 discloses an antireflection film comprising a substrate having formed thereon a high-refractive layer and a low-refractive layer in this order. The antireflection film can further comprise a medium-refractive layer between the substrate and the high-refractive layer. The low-refractive layer is formed by applying a polymer or inorganic particles by wet coating.
JP-A-2-245702 discloses an antireflection film containing two or more kinds of ultrafine particles (e.g., MgF2 and SiO2). The mixing ratio of the particles are varied in the film thickness direction to vary the refractive index thereby achieving optical properties similar to those of the antireflection film of JP-A-59-50401 having a high-refractive layer/low-refractive layer combination. The ultrafine particles are bound by SiO2 resulting from thermal decomposition of ethyl silicate. Thermal decomposition of ethyl silicate is accompanied by combustion of the ethyl moiety to generate carbon dioxide and steam, which release from the coating layer to leave interstices among the ultrafine particles as shown in FIG. 1 of the publication.
JP-A-5-13021 proposes filling the above-described interstices among ultrafine particles with a binder. JP-A-7-48527 discloses an antireflection film containing a particulate inorganic substance comprising porous silica and a binder.
JP-A-11-6902 teaches an all-wet coating technique for making an antireflection film with high film strength and low reflection at low cost, disclosing a film having a three-layered antireflection layer formed by wet coating, in which at least two inorganic particles are stacked on a low-refractive layer to form a layer containing microvoids.
On the other hand, known means for imparting antiglare properties to an antireflection film formed by a wet coating technique include coating a substrate having surface unevenness with an antireflection layer, incorporating matting particles for making surface unevenness into an antireflection layer, and embossing a smooth antireflection film to form surface unevenness, as described in JP-A-2000-275401 and JP-A-2000-275404.
An antireflection film formed by vacuum deposition or sputtering which has an average reflectance of 0.4% or lower in a wavelength region of 450 to 650 nm gives reflected light strongly tinged in reddish to bluish purple. Where a light source is behind a viewer, such tinged reflected light impairs display quality. On the other hand, an antireflection film formed by wet coating, while having reflected light of nearly neutral color, has an average reflectance more than 1%, which is insufficient particularly in applications where outside light is incident directly on the display face. Example 24 of JP-A-11-6902 supra reads that the film formed by wet coating had an average reflectance of 0.35%. However, the reflected light from the film turned out to assume an intense reddish purple color as calculated from the reflection spectrum. A sample actually prepared by the present inventors according to the disclosure was found to have reflected light intensely colored in reddish purple when observed with the naked eye, which ruins the display quality.
An object of the present invention is to provide an antireflection film having a low reflectance and giving non-tinged reflected light.
Another object of the invention is to provide a polarizer having the antireflection film and an apparatus for displaying an image using the antireflection film or the polarizer.